Batteries can’t solve the world’s biggest energy-storage problem. One startup has a solution.

Batteries can’t solve the world’s biggest energy-storage problem. One startup has a solution.

Sometimes, there can be too much of a good thing.

Every so often, from California to Germany, there’s news of “negative electricity prices,” a peculiar side effect of global efforts to generate clean energy. Solar farms and wind turbines produce varying amounts of power based on the vagaries of the weather. So we build electrical grids to handle only the power levels we expect in a given location. But in some cases, there’s more sun or wind than expected, and these renewable energy sources pump in more power than the grid can handle. The producers of that power then have to pay customers to use up the excess electricity; otherwise, the grid would be overloaded and fail.

As we build more and more renewable-power capacity in efforts to meet the emissions-reduction goals of the Paris climate agreement, these situations will become more common. Startups led by entrepreneurs who see this future on the horizon are now looking for ways to make money off the inevitable excess clean electricity.

On a mildly chilly day in April, with the smell of poo in the air, I met one of these startups at a sewage-water treatment plant in Copenhagen, Denmark. Electrochaea takes carbon dioxide produced during the process of cleaning wastewater, and converts it into natural gas. That alone would be impressive enough; if we want to stop global warming in its tracks, we need to do everything we can to keep CO2 from entering the atmosphere. But Electrochaea has also figured out a way to power the whole enterprise with the excess green energy produced during particularly sunny and windy days that otherwise would have gone to waste, because there would have been no way to store it.

In other words, when scaled up, Electrochaea’s process could be an answer to one of the biggest problems of the 21st century: energy storage, while also making a dent in cutting emissions.

The battery problem

The biggest problem with wind and solar energy is that they’re intermittent. There might be violent winds one day, and calm skies the next; broiling sunshine on Monday and 100% cloud cover on Tuesday. Some argue this problem is easily overcome by storing any excess energy in batteries until it’s needed at a later time. Further, battery advocates say, even though the bookcase-sized batteries required to store solar energy for a small home are expensive today, prices are falling and will continue to fall for some time.

Except it’s not that easy. The batteries on the market for these applications are, essentially, large versions of the lithium-ion batteries found in mobile phones. They can only store energy for a certain amount of time—weeks, at most. As soon as the charging source is removed, they start to lose the charge.

That’s not a problem if the batteries are for ironing out the peaks and troughs of daily use. The trouble is that humanity’s energy demand is skewed based on local seasons, which requires sometimes drawing on every available source, and sometimes not using much energy at all. Mumbai’s peak energy demand is during the hottest days of summer, when people run air conditioners to survive. London’s peak energy demand comes during the coldest days of winters, when people burn natural gas to heat their homes and offices.

Peak energy demand, whether for heating or cooling, can be as much as 20 times the energy consumed on an average day. Today, we shovel more coal or pump more natural gas into fossil-fuel power plants on those high-demand days. Some places, like Bridgeport in Connecticut, have old fossil-fuel plants, often coal, that they keep shut down most of the year and fire up only during peak demand. Obviously, that won’t work in a future powered by renewable energy.

There are two solutions on the table for inter-seasonal energy storage, and they both involve massive investment in infrastructure: First, you could build so many solar panel fields or so many wind turbines that you could produce much more than 20 times the power of an average day. The upshot: you’d have much more excess energy on a low-demand day, but would at least be able to fill demand on peak-demand days. The second option is to get so many batteries that they can store up enough excess energy that, even as they lose their charge, there’s still enough power to get the grid through peak-demand days.

Even if both renewable generation and storage were affordable enough for these plans—and they’re not, yet—there’s still another economic wall that might be impossible to traverse: Most of the time, your new gigantic power plant and fleet of batteries would be useless, because peak demand happens only a few times each year. No government can waste the money needed to build something with so little utility.

Store it another way

Beyond batteries, there are other mechanical ways to store energy. One is to pump water into elevated lakes. Another is to compress air with excess energy. Yet another is to store energy in the form of a high-speed rotating disk. But, like batteries, none of these options can store energy between seasons.

There is one option for the inter-seasonal problem called underground thermal-energy storage. It works on a simple principle: no matter the temperature above ground, at a depth of about 15 meters, temperature in most places on Earth is about the same: 10°C (or 55°F). The planet’s soil provides natural insulation, and, in theory, we could use that insulation to store energy.

There have been successful pilot projects around the world showing you can set up solar panels that, after filling the grid, use any excess electricity to heat gravel, heat-carrying chemicals, or water stored in tanks deep underground. With enough insulation, the heat could be stored for months, until it’s needed in homes nearby, and delivered to them via pipes and heat pumps. (This heat energy can also be converted to run air conditioners, where cooling is needed instead of heating.)

There are just two problems when it comes to scaling up: First, it’s expensive to build. Even if the cost of construction and management were to come down, if cities and towns haven’t already planned for underground reservoirs (and most haven’t), then it can be prohibitively exorbitant to find and secure the space. Second, the solution only works at a local scale, because transporting heat comes with natural losses. So the farther you need to move it from the storage site, the more loss you have to deal with.

Electrochaea provides another option, where renewable energy could be stored indefinitely and transported without losses.

The green goo

If you don’t mind the smell, wastewater treatment plants are fascinating. The Copenhagen plant takes in all the water sent down toilets, bathrooms, and kitchen sinks, and puts out H2O that’s nearly clean enough to drink—just one more step would be required, the plant operator told me. But since the city has no shortage of water, the treatment plant dumps its clean, but non-potable water into the North Sea.

Before that, the water goes through dozens of steps, including one where organic matter is allowed to settled to the bottom of large, open tanks. This sludge, rich in carbon-containing molecules, is transferred to a sealed bioreactor where microbes filtered from local soil are added. If this were done in the open tanks, the microbes would break the matter down slowly to produce carbon dioxide. But, in the bioreactor, in the absence of oxygen, a different set of microbes springs into action. They produce methane—the primary component of natural gas—instead.